1. Technical Field
The present invention relates to a rotational-linear motion converter utilizing magnetism.
2. Description of Related Art
Hitherto, as a typical rotational-linear motion converter, a device to which a so-called ball screw mechanism is applied is used. In this type of rotational-linear motion converter, since friction is generated between a ball and thread grooves, it is likely that noise or vibration will occur, thereby making it difficult to enhance the longevity of such a converter. Additionally, in this type of rotational-linear motion converter, regular maintenance, such as greasing, is required for reducing the occurrence of noise or vibration caused by aging or for minimizing the wear of a ball. In this manner, the possibility of scattering of grease or the maintenance has to be considered, and thus, installation places of this type of rotational-linear motion converter are restricted, thereby decreasing the design flexibility.
These days, attention is being focused on technologies concerning, for example, non-contact rotational-linear motion converters utilizing magnetism, which may overcome the above-described drawbacks unique to a rotational-linear motion converter using a ball screw mechanism.
Japanese Unexamined Patent Application Publication No. 2008-215429 discloses a magnetic power transmission device which includes a non-contact magnetic rack and pinion mechanism and which converts rotational motion to linear motion. This magnetic rack and pinion mechanism includes a shaft-like member and a pair of support plates. The shaft-like member has permanent magnets which are spirally formed on the outer peripheral surface of the shaft-like member with predetermined pitches. The pair of support plates has permanent magnets on the internal surfaces thereof with the same pitches as those of the permanent magnets of the shaft-like member so that these permanent magnets may oppose the permanent magnets of the shaft-like member. In this magnetic rack and pinion mechanism, due to a magnetic attractive force generated between the permanent magnets of the shaft-like member and the permanent magnets of the support plates which oppose each other, rotational motion of the shaft-like member is converted into linear motion of the support plates. The strength of the magnetic attractive force differs depending on the number of opposing permanent magnets, and the magnetic attractive force becomes stronger as the number of opposing permanent magnets is greater. As the magnetic attractive force becomes stronger, the permissible thrust of the magnetic rack and pinion mechanism also becomes greater.
Japanese Unexamined Patent Application Publication No. 2007-215264 (page 11, FIG. 10) discloses an actuator which includes a non-contact magnetic velocity-reduction drive and which converts rotational motion to linear motion. The non-contact magnetic velocity-reduction drive includes a base, a drive head, and a permanent magnet. The base is constituted by a magnetic body having projecting portions and recessed portions which are alternately disposed at predetermined intervals on the top surface of the base. In the drive head, a magnetic circuit is formed between an internal space of the drive head and a base opposing surface which opposes the top surface of the base. The permanent magnet is rotatably fitted in the internal space of the drive head. In this magnetic velocity-reduction drive, a closed magnetic field which passes through the drive head and the base is formed due to the presence of the permanent magnet. The closed magnetic field is formed along the path of the magnetic circuit of the drive head and the top surface of the base due to the presence of a magnetic flux passing through an area where the magnetic circuit of the drive head opposes the projecting portions of the base. In this magnetic velocity-reduction drive, by a rotating magnetic field (closed magnetic field) generated by rotating the permanent magnet, the thrust in the horizontal direction (restoration force of a magnetic field) is obtained, and then, rotational motion of the permanent magnet is converted into linear motion of the base (in this actuator, since the base is fixed, the drive head, which is movable, actually performs linear motion).
The strength of the thrust of the rotating magnetic field differs depending on the magnetic field intensity (magnetic flux density or the number of magnetic lines of force per unit area). The thrust of the rotating magnetic field becomes stronger as the magnetic field intensity becomes greater. If the thrust of the rotating magnetic field is strong, the permissible thrust that can be transmitted between the drive shaft of the rotating magnet and the base is increased. Accordingly, in order to generate a large permissible thrust, it is necessary to increase the number of magnetic lines of force per unit area which forms a rotating magnetic field.
In the magnetic power transmission device disclosed in Japanese Unexamined Patent Application Publication No. 2008-215429, since permanent magnets are disposed both on the shaft-like member and the support plates, if a long moving distance (stroke) of the support plates which perform linear motion is required, it is necessary to increase the number of permanent magnets or to increase the pitch between the permanent magnets disposed on the shaft-like member. If many permanent magnets are disposed, the shaft-like member is likely to sag, thereby decreasing the positioning precision. Under the current situation, a low-cost, high-precision magnetic power transmission device is demanded. Thus, if a long stroke is required, it is difficult to use the magnetic power transmission device disclosed in this publication.
If the pitch between the permanent magnets is increased, the number of permanent magnets disposed on the shaft-like member which oppose the permanent magnets of the support plates is decreased. Accordingly, the permissible thrust of the magnetic rack and pinion mechanism is decreased. Thus, if a large permissible thrust is desired, it is also difficult to use the magnetic power transmission device disclosed in this publication.
In the magnetic velocity-reduction drive disclosed in Japanese Unexamined Patent Application Publication No. 2007-215264, the magnetic field intensity of the closed magnetic field is determined by the number of magnetic lines of force passing through the area where the magnetic circuit of the drive head opposes the projecting portions of the base. Among these magnetic lines of force, there are some magnetic lines of force which pass through an area where the magnetic circuit of the drive head does not oppose the projecting portions of the base. However, such magnetic lines of force produce very little influence on the magnetic field intensity of the closed magnetic field.
Accordingly, in the magnetic velocity-reduction drive disclosed in this publication, in order to increase the permissible thrust, it is necessary to increase the area by which the magnetic circuit of the drive head and the projecting portions of the base oppose each other. Thus, the size of the magnetic velocity-reduction drive is inevitably increased. Under the current situation, a small magnetic velocity-reduction drive mechanism is demanded. Thus, it is difficult to use the magnetic velocity-reduction drive disclosed in this publication.
If the area by which the magnetic circuit of the drive head and the projecting portions of the base oppose each other is increased while maintaining a small size of the magnetic velocity-reduction drive, the pitch of the projecting portions of the base has to be increased. Then, the number of projecting portions of the base that can be disposed within the same length of the base is decreased, which makes it difficult to increase the acceleration reduction velocity ratio of the magnetic velocity-reduction drive mechanism. Accordingly, if a large acceleration reduction velocity ratio is desired, it is also difficult to use the magnetic velocity-reduction drive disclosed in this publication.